Modern-day electronic systems often require voltage regulators which receive an unregulated line voltage and provide a regulated power supply voltage to an electrical load. Such voltage regulators are required to provide a supply voltage having a relatively constant magnitude to the electrical load even though the resistance of the electrical load changes and even though the magnitude of the line voltage changes. The magnitude of the regulated output voltage is less than or equal to the lowest magnitude of the line voltage and greater than the magnitude of a fixed reference voltage which can be provided by a zener diode, a Brokaw, or a bandgap reference.
More particularly a common configuration of a prior art series voltage regulator includes PNP Darlington pass transistors having a composite emitter electrode connected to the line voltage and collector electrodes connected to the regulator output terminal. A differential error amplifier includes one transistor having a collector connected to the composite base of the Darlington transistors and a base electrode connected to the voltage reference supply. Another differential transistor having a collector connected to the regulator output terminal is also included in the error amplifier. A bias supply current source is connected between the line voltage terminal and the collector electrode of the first mentioned differential amplifier transistor. A differential amplifier current sink or supply is connected to the emitters of the differential transistors.
Unfortunately, PNP Darlington transistors commonly used in monolithic integrated circuits for regulating a positive voltage supply have low betas. Consequently, the current sink for the differential error amplifier is required to draw or sink a current having an undesirably large magnitude under quiescent or no load conditions so that a desired amount of drive can be provided to the Darlington under full load conditions. Quiescent current also must be conducted by the bias current supply to facilitate high frequency response. This large quiescent current is disadvantageous in at least two respects. Firstly, the quiescent current drain wastes energy and, secondly, the large quiescent current must be dissipated by the regulator thereby undesirably heating the die.
Another problem with the foregoing standard prior art series voltage regulator relates to voltage breakdown. More specifically, the magnitude of the line potential minus the voltage drop across the emitter-to-base junctions of the Darlington is present at the collector electrode of the first mentioned differential transistor. Furthermore, the reference voltage, which for the Brokaw or bandgap reference generators is approximately 1.2 volts, is applied to the base electrode of the same differential error amplifier transistor. Accordingly, the magnitude collector-to-base voltage on the differential transistor is approximately equal to the magnitude of the input line voltage minus only a few volts. In automotive applications, the line or battery voltage supplied by the automobile may be as much as 50 volts during a "load dump" condition, which occurs when one of the battery cables is lifted while the electrical system is supplying a current having a large magnitude. Thus, the differential transistor is required to withstand collector-to-base voltages of at least 50 volts. Such transistors are difficult to fabricate by known I.sup.2 L compatible processes in a monolithic integrated circuit when connected in the prior art configuration. I.sup.2 L processes are commonly used for fabricating circuitry used in automotive applications.